Thermal Heat Exchanger Research Articles

A New Experimental Methodology to Study Convective Heat Transfer in Oil Jet Lubricated Gear Units

The purpose of this study is to generate some experimental data associated with the thermal heat exchange between an oil jet flow and a rotating gear. To this end, a specific test bench was designed. The principle of this test bench is to inject oil heated to a temperature of about 80 °C onto a rotating test sample at ambient temperature. Temperature measurements of the oil via injection nozzles and the rotating component allow the determination of the heat flow between these elements using a numerical method developed to this end. This test rig enables the study of the parameters that may affect heat exchange, such as oil flow rate and injection temperature, nozzle geometry and position, gear rotational speed and tooth geometry, or oil characteristics. In this study, three of these parameters were investigated, namely the test sample rotational speed, the oil flow rate, and the oil jet velocity. The experiments were conducted on an aluminum disc and spur gear. Subsequently, the experimental results were compared with existing models that represent the convective exchanges between oil and a gear. Some discrepancies between existing models and experimental results appear at high rotational speeds, underlining that the convective heat transfer does not always increase with this parameter.

A theoretical approach to be applied in heat exchangers by using the thermal efficiency concept and the second law of thermodynamic

A review of the concepts of thermal efficiency and thermal and hydraulic irreversibilities is presented, applying the second law of thermodynamics and thermodynamic Bejan number. An example problem, typical of thermal heat exchange between two fluids, is given, with a dimensionless solution for parallel and counterflow flows. The quantities of interest presented through the example are thermal efficiency, thermal effectiveness, thermal irreversibility, the relationship between outlet and inlet temperatures versus the number of thermal units, and outlet temperature for hot fluid. The theory presented in this review has been applied to numerous problems related to heat exchangers over the last three years, as per references.

A salt based integrated thermal store and heat exchanger for CAES systems

A salt based integrated thermal store and heat exchanger for CAES systems

Application of Lévêque Analogy to Open Cellular Porous Materials

Open-porous materials are used for thermal management applications, catalyst support, heat exchangers, and others owing to their enhanced heat transfer. Designing porous material for a specific heat transfer application requires quantifying effective thermal conductivity, pressure drop, and heat transfer as computed data or experimental correlations. This work extends the Lévêque analogy proposed for calculating heat transfer performance from pressure drop data for tube bundles and cross-rod matrices to porous materials. The current work shows the applicability of this analogy for periodic open-cellular porous materials. Seven unit cell geometries representing periodic cellular structures with 75% - 95% porosity are considered. Periodic flow and heat transfer simulation is performed in ANSYS Fluent for Reynolds numbers between 1 and 100 to provide pressure drop and heat transfer results. Permeability and inertial coefficient are estimated, and permeability is correlated successfully with the porosity and fiber diameter of the unit cell geometries. This correlation is validated using experimental data available in the literature, which allows for predicting pressure drops directly from the geometry. Lévêque’s analogy in the original form is explored using the pressure drop coefficient and Nusselt number, which provides a 44% frictional contribution to the total pressure drop. The correlation for the current geometries is further improved using a modified version of the Lévêque analogy, where the exponent is estimated using best fit instead of fixing at 1/3. The modified correlation is validated using experimental data from the literature, and a guideline to predict pressure drop and heat transfer from geometry is provided for periodic open-cellular porous materials.

Procedure for simulating radiators of liquid cooling systems of avionics

Setting the problem. Analysis and assessment of the thermal mode of radioelectronic equipment (RES) in modern radioelectronic equipment is the most important task that is solved at the stage of design and development of the future device. Therefore, the choice of cooling system for heat-loaded structures is an integral component that must be solved in the early stages of design. The use of a modern set of computer-aided design tools (CAD) throughout the entire process of creating RES allows you to take into account all possible problem areas without resorting to the manufacture of a layout and prototype. Modeling of thermal characteristics and heat exchange processes provides for modeling procedures and ensuring the thermal regime mainly at the stage of design and topological design. At the same time, the analysis of thermal characteristics, as a rule, carried out after solving any design synthesis problem (placement, layout, selection and development of block structures, racks, etc.), if the modeling results are unsatisfactory, the design changes. Thus, in order to reduce time costs and reduce the iteration of the design process, it is necessary to solve the issues of ensuring the thermal regime in the early stages. Most often, the problem can be solved by increasing the area of the heat-emitting surface (radiator), but in some cases there is a need for additional cooling, in our case, additional use of liquid cooling. This is the reason for the relevance of the development of the methodology for modeling radiators of liquid cooling systems of the RES. Purpose. Present the methodology for simulating radiators of liquid cooling systems of the RES, the reliability of which could be confirmed by the results of analytical calculations and data of a full-scale experiment. Results. A mathematical statement has been formed, a modeling technique has been proposed and procedures for optimizing the cooling system have been implemented. Based on the developed 3D model of a radiator with a liquid cooling system, multivariable analysis procedures are implemented using modern computer-aided design tools. The liquid cooling system was optimized using modern software modeling tools, in particular the Solidworks Simulation program.

A Novel Numerical Method for Geothermal Reservoirs Embedded with Fracture Networks and Parameter Optimization for Power Generation

Geothermal recovery involves a coupled thermo-hydro-mechanical (THM) process in fractured rocks. A fluid transient equilibrium equation, considering thermal conduction, convection, and heat exchange, is established. The evolution of the reservoir permeability and the variance in the fracture aperture due to a change in the stress field are derived simultaneously. THM coupling is accomplished through iterative hydromechanical and thermo-hydro processes. To overcome the difficulty of geometric discretization, a three-dimensional THM coupler model embedded with discrete fracture networks, using a zero-thickness surface and line elements to simulate fractures and injection/production wells, is established to evaluate the geothermal production. The reliability of the method is verified by a case study. Then, this method is applied to evaluate the influence of the geometric topological characteristics of fracture networks and the fracture aperture on the reservoir temperature evolution and heat extraction effectiveness. The results show that the power generation efficiency and geothermal depletion rate are significantly affected by the injection–production pressure. Injection wells and production wells with pressures higher than the initial fluid pressure in the fractures can be used to significantly increase power generation, but the consumption of geothermal energy and loss of efficiency are significant and rapid. To achieve better benefits for the geothermal recovery system, an optimization algorithm based on simultaneous perturbation stochastic approximation (SPSA) is proposed; it takes the power generation efficiency as the objective function, and the corresponding program is developed using MATLAB to optimize the position and pressure values for each production well. The results show that the heat transfer for the entire EGS reservoir becomes more uniform after optimization, and the heat transfer efficiency is greatly improved.

Experimental validation of the dynamic thermal network approach in modeling buried pipes

The transient behavior of buried pipe systems plays a significant role in many heating and cooling systems, particularly in thermal energy networks and ground heat exchangers. In this study, the dynamic thermal network (DTN) approach’s validity as a response factor method in modeling dynamic conduction heat transfer in a buried pipe system is experimentally validated. A lab-scale representation of a buried pipe system has been excited by step changes in boundary temperatures and heat fluxes measured up to times approaching steady-state conditions. This data is used to derive weighting factors and also evaluate the validity of numerical representations of the buried pipe and to verify that the DTN method can reproduce the heat flux responses. It is demonstrated that the weighting factors required in this method can be derived from both numerical and experimental step-response time series data. The DTN method is found to be both accurate in reproducing the heat fluxes in the validation experiments but also significantly more computationally efficient than a conventional numerical model when simulating long timescale responses in buried pipe systems.

A Simulation Study of Solar Thermal Steam Generation for Thermal Recovery of Thick Oil Based on Aspen Plus

High-temperature steam injection is a common means of thermal recovery of thick oil. Although the steam injection boiler currently used for thick oil injection can achieve fast start-stop control, the power consumption of the pump is large, and the boiler control is demanding. Besides, a large amount of fossil fuel is needed to supply the heat required for steam heating. Using solar thermal to realize the transformation of solar energy-molten salt heat energy-steam energy can effectively reduce the burning of fossil fuels and realize clean thermal recovery of thick oil. To ensure the actual demand of thermal recovery of thick oil, it is necessary to determine a reasonable steam evaporation volume and vapor phase fraction. This paper simulates a solar thermal heat exchange system for molten salt-steam based on Aspen Plus. The simulation results show that the heat load and required solar panel area of all heat exchangers rise when the evaporation volume increases. However, the superheater grows most slowly. When the vapor phase fraction at the evaporator outlet rises to 1, there is no phase change heat transfer in the superheater, and its heat load plummets to 230 kW.

In-situ evaluation of window-wall joint performance using numerical models and thermal images

Infrared thermography is an effective method for assessing the on-site thermal performance and heat exchange of structures. On-site infrared thermography was generally performed under the assumption of quasi-steady sate, which refers to the averaged boundary conditions; however, a description of the actual on-site dynamic conditions is necessary to increase the reproducibility of the assessment. This study, through simulation models, describes the infrared thermal images of the surrounding wall depending on the thermal performance at the joint between the window and wall, through which the thermal performance of the joint is estimated to quantify insulation defects or thermal bridge flaws. As incremental research to the previous chamber-based preliminary research, the main objective was to apply the assessment method to an on-site experiment and verify the reproducibility of the results. In the case of the distinct differentiation of the temperature distribution of the wall according to the material properties of the joint, the reproducibility of the assessed results was verified as values of the material properties of the joint of the physical model that represent the minimum error were the same, regardless of the period of assessments. Furthermore, the difference in the results from the physical model, in which the derived physical property values were applied, and the infrared thermography measurements were also within 5% and 20% (coefficient of variance of the root mean square error and normalized root mean squared error).

Thermodynamic, economic and environmental assessments of a novel solar-driven combined cooling and power system

The present study investigates an improved design of a solar-driven combined cooling and power system based on energy, exergy, economic and environmental perspectives. It includes evacuated tube collector, thermal energy storage, vapor generator, condenser, evaporator, ejectors, turbine, pump, heat exchanger and expansion valve. It is compared with the same capacity's individual cooling and power system. Effects of operating temperatures, component efficiencies, seasonal climate change and cooling-to-power ratio are also investigated. At the typical operating conditions with isobutene, the novel cycle has a performance index of 0.08 and overall exergy efficiency of 3.05%, which are nearly 33% and 44% higher than the individual cooling and power system, respectively. The specific total cost and equivalent mass of CO2 emission are also lower than the individual cooling and power system by 38% and 26%, respectively. At the higher vapor generator and evaporator temperatures but lower ambient temperature, the performance index is higher, but the specific total cost and equivalent CO2 emission are lower. The performance index and exergy efficiency are maximum for January and minimum for April, whereas specific cost and CO2 emission are minimum for January and maximum for April. The performance index is better, but the specific total cost is lower at a higher cooling-to-power ratio.

NEW SOLUTIONS FOR MAKING ELECTRIC MACHINES WITH HIGH SPECIFIC POWERS FOR PROPULSION SYSTEMS OF ELECTRIC AND HYBRID VEHICLES

Within the propulsion systems of electric or hybrid vehicles, road, naval or air, electric cars represent some of the main components that determine the weight of these systems. As a result, the realization of light electric propulsion systems requires the use of electric machines with high specific powers. In this work, we will present the results published in recent years in the specialized literature regarding the realization of electric car cooling systems in order to obtain high specific powers. New technologies for dissipating the heat generated by the energy losses in these machines will be presented, technologies that combine both passive cooling structures and active cooling structures, such as those using heat pumps. Also, research results will be presented regarding the realization of some components of these cooling structures using additive manufacturing technology, made with 3D printing systems, a technology that has evolved rapidly in recent years offering a new way to easily create, from various conductive, insulating or magnetic materials of parts with complex geometry, such as electric machine windings with direct incorporation of ceramic insulation, magnetic circuits with permanent magnets or thermal heat exchanger circuits.

A review of current developments in the use of materials with latent heat phase changes for the storage of thermal energy

Phase change materials (PCMs) are popular because they can store and release latent heat for energy saving and thermal management. Despite PCMs' latent heat and melting temperatures variety, their low thermal conductivity (TC) limits their usage. This problem and attempts to overcome the poor heat conductivity of PCMs are discussed in length in this publication. High TC nanoparticles, encapsulation and porous metal foams are solutions for the problem. PCMs may be used to store and release energy in electrical gadgets, photovoltaic panels, desalination solar stills, thermoelectric generators, textiles, air conditioning, battery thermal management, water heating, food packaging and heat exchangers. In this review we discuss the different methods which are being used to increase the thermal conductivity.

Integration design and control strategy of sCO 2 Brayton cycle for concentrated solar power system

The concentrated solar power (CSP) system integrated with supercritical carbon dioxide (sCO 2) Brayton cycle is considered as the major development trend of clean energy technology in mitigating climate change and promoting sustainable energy owing to its more compact structure, higher efficiency and lower cost. While the system integration and control are more complex because the thermal energy storage and heat exchange units are obliged to be provided with capacity and property matching with sCO 2 compression and expansion units. Therefore, there is an urgent need for the whole system modeling and dynamic simulation of sCO 2-CSP plants. In the paper, running logic of the integrated system is determined and novel system dynamic simulation platform is built. Moreover, for the sake of optimizing the system design, different control strategies are further studied. When the extremum-seeking control strategy is used, the average daily generating capacity of the system is 2.06% higher than that of the traditional fixed inventory control strategy; and when iterative learning control strategy is used, the average daily generating capacity of the system is 2.15% higher than that of the traditional fixed inventory control strategy. The progress of the technology will promote the development of new power systems and early realization of carbon neutrality.

Simplified dynamic model of a nuclear reactor

The article is devoted to the issues of nuclear energy. It considers a simplified dynamic model simulating the main processes occurring in a nuclear power plant. The basis of the proposed model is a model of heat release in the reactor, simulating the relative neutron power and temperature of the coolant in the hot thread of the circulation circuit. The position of the absorbing assemblies, which determines the reactivity introduced by them, is formed in an operational control model that simulates bringing the assemblies into a pre-start state, displacement of the control assembly under the influence of a power regulator, as well as under the action of protections of various levels. The purge/recharge model describes the dynamics of the concentration of boron absorber during water exchange between the deaerator and the circulation circuit. Models of thermal deaeration, pressure compensation and heat exchange in the circuit simulate the pressure and temperature of the coolant.

Microporous Materials for Hydrogen Liquefiers and Storage Vessels

Microporous Composite Materials, made of metal oxide ionic fragments inserted in carbonaceous substrates, might be used as intermediates in hydrogen phase transitions, allowing progressive switch between their gas and liquid states. When crossed by an hydrogen flow these composite materials combine adsorbing, magnetic, catalytic and thermal properties efficient in hydrogen devices, liquefiers or storage vessels. Liquefaction and storage are complementary processes in practice, although inverse in principle. Plate-fin devices filled with composite materials can conjugate many thermal functions, heat exchangers, catalytic converters and pressure expanders that are necessary stages in large-scale liquefiers. Break-even storage times permit operation of the liquefaction unit at optimum ortho-para conversion thus yielding minimum practical work to be furnished. A barrage-system of successive porous plugs inserted in Hydrogen vessels would regulate the flow rate by J-T expansions, convert part of the environmental heat in rotational energy and confer stability to the system by damping the fluctuations. Consequently storage vessels of short dormancy period might be manufactured to be of low weight and low cost. After a short review of existing devices, a logical scheme interconnecting physical, economic and technologic challenges suggests that activated carbon materials of cage-like porous structures would promote and accelerate the development of storage vessels in hydrogen transport vehicles and facilitate the liquid hydrogen production by regulating the hydrogen streams in the industrial liquefiers.

On a modification of the Hamming method for summing discrete Fourier series and its application to solve the problem of correction of thermographic images

The paper considers mathematical methods of correction of thermographic images (thermograms) in the form of temperature distribution on the surface of the object under study, obtained using a thermal imager. The thermogram reproduces the image of the heat-generating structures located inside the object under study. This image is transmitted with distortions, since the sources are usually removed from its surface and the temperature distribution on the surface of the object transmits the image as blurred due to the processes of thermal conductivity and heat exchange. In this paper, the continuation of the temperature function as a harmonic function from the surface deep into the object under study in order to obtain a temperature distribution function near sources is considered as a correction principle. This distribution is considered as an adjusted thermogram. The continuation is carried out on the basis of solving the Cauchy problem for the Laplace equation - an ill-posed problem. The solution is constructed using the Tikhonov regularization method. The main part of the constructed approximate solution is presented as a Fourier series by the eigenfunctions of the Laplace operator. Discretization of the problem leads to discrete Fourier series. A modification of the Hamming method for summing Fourier series and calculating their coefficients is proposed.

Evaluation of groundwater pumping impact on the thermal conductivity of neighboring ground source heat exchangers

This study evaluates the correlation between groundwater velocity and the thermal conductivity and heat exchange performance of the ground source heat exchangers. The groundwater was extracted from un-grouted water well, which is located beside the drilled ground source heat exchangers (GHEs), to make artificial groundwater velocity in the region. The thermal response tests were carried out for three GHEs, which were drilled around the water well at different distances at the test site in Yamagata City, Northern Japan. During the tests, the pumping rate varied from 0 (no pumping) to 200 L/min in three steps and at two days intervals. The hydrothermal parameters of the ground around the GHEs were estimated for the GHEs and each step of pumping rate using a script in MATLAB software and with the help of the moving line source (MLS) method. This method can estimate the groundwater velocity and the effective thermal conductivity of the soil for different pumping rates to study the effect of the pumping on the neighboring GHEs. As a result, the thermal conductivity was improved by pumping groundwater in all GHEs. The rate of pumping had a direct relation with the improvement level and higher rate of pumping caused higher amount of thermal conductivity improvement. Moreover, this study showed that the location of the GHE around the pumping point does not effect on the level of thermal conductivity increment. Therefore, there was no correlation between the distance of the GHE and pumping well and the closest GHE was not necessarily experienced highest impact.

Design Optimization, Thermohydraulic, and Enviro-Economic Analysis of Twisted Perforated Tape Insert-Based Heat Exchanger With Nanofluid Using Computational Fluid Dynamics and Taguchi Grey Method

Abstract In this paper, the effect of perforated twisted tape insert (PTTI)-based heat exchanger (HX) utilizing nanofluid as working fluid undercooling, turbulent flow model has been investigated numerically. Parameters, i.e., nanofluid mass flow rates (0.018–0.038 kg/s), perforated pitches (20 mm–40 mm), and perforated diameters (2 mm–4 mm) variation effects on fluid outlet temperature, Nusselt ratio, Friction ratio, pressure drop, overall thermal performance, CO2 discharge, and heat exchanger operating cost (HXOC) have been investigated. This work also focuses on design optimization with three different factors and three levels for higher heat transfer coefficient (HTC) and minimum pressure drop based on the Taguchi–Grey method. Computational fluid dynamics (CFD) output is used as an input value for statistical analysis. Results revealed that the PTTI in HX successfully achieved overall heat transfer enhancement in the range of 19.2% to 28.5%, but at the cost of pressure penalty of 126% to 163% higher than the plain tube. Critical Reynolds number 7927, above which PTTI in HX is least suitable for heat transfer enhancement as fluid velocity dominates over heat transfer and 1.7–2.5 times higher carbon discharge to the environment and HXOC. Preference sets of geometrical and fluid parameters are obtained using Grey analysis. Based on statistical analysis, in the considered levels, a group of parameters to attain higher HTC and minimum pressure drop are mass flow rate of 0.018 kg/s, a perforated pitch of 20 mm, and a perforated diameter of 4 mm.

Enhanced Heat Transfer Using Oil-Based Nanofluid Flow through Conduits: A Review

The application of nanofluids for enhancing the heat transfer rate is widely used in various heat exchanger applications. The selection of oil as the base to prepare nanofluids significantly enhances the thermal performance, due to its high heat carrying capacity as compared to conventional base fluid. A review is performed of various heat exchanger conduits having base fluid as nanoparticles with oil. It is reported that the heat transfer rate of a heat exchanger is significantly increased with the use of oil-based nanofluids. The rate of heat transfer depends on the type of nanoparticle, its concentration and diameter, the base fluid, as well as factors like the mixture of more than two nanoparticles (hybrid nanofluids) and stability. A review is also performed of the thermal performance of the different nanofluids analyzed by various investigators. The heat transfer system reviewed in this work includes triangular, square, and circular conduits, as well as rib surface conduits. The review of various applications viz. solar thermal systems, heat exchangers, refrigerators, and engines, is carried out where the inclusion of the oil base is used. It is reported that the amalgamation of the nanomaterial with the oil as base fluid is a prolific technique to enhance thermal performance. The performance of the reviewed research work is comparatively analyzed for different aspects viz. thermal oil, mineral oil, hybrid, and conventional nanoparticles, concentration of nanoparticles, etc. The novelty of the present work is the determination of the effective performing oil-based nanofluid in various applications, to figure out the selection of specific mineral oil, thermal oil, nanoparticle concentration, and hybrid nanofluids.

Optimizing phase change composite thermal energy storage using the thermal Ragone framework

Optimal design of thermal energy storage devices can allow systems to approach specific design objectives while maintaining desired levels of performance. The thermal Ragone framework has been applied to the design of thermal energy storage heat exchangers, specifically identifying relationships between their power and energy capabilities. In this paper, a finite-difference model is used to optimize thermal storage heat exchanger designs for three objectives given a discharge power constraint. The three objectives are maximizing energy density, minimizing energy-specific capital costs, and minimizing the levelized cost of storage. This study focuses on the design of planar thermal energy storage heat exchangers with phase change materials and thermal conductivity additives. Key design parameters identified included the conductivity additive volume fraction, spacing between heat transfer fluid tubes, and the phase transition temperature. The optimal design is found to depend strongly on the required thermal power, with higher powers requiring transition temperatures further from the use temperature, more conductivity additives, and closer tube spacing. There is also a trade-off between these design parameters, where changing one (e.g., closer tube spacing) can allow another parameter to be relaxed (e.g., less conductivity additive). To aid in future device design, models of reduced complexity were developed and evaluated for their ability to predict optimal designs. These simplified models can predict the performance of thermal energy storage heat exchangers up to 5000 times faster than the finite-difference model. For the design objectives of maximizing energy density, minimizing costs, and minimizing levelized cost of storage, the simplified models predicted optimal designs with average absolute deviations below 2 % in relation to the optimal designs chosen by the finite difference model optimization. These models can serve as tools to further study thermal storage heat exchangers incorporating the realistic trade-off between the device's power and energy.